In communication and application processes, there is often a need for time-keeping circuitry that is active while most other processes are in a “sleep” mode. For conservation of battery life and other factors, it is desirable that such time-keeping circuitry use only small amounts of power. As one example, a microprocessor in a mobile telephone or personal digital assistant often requires a very low power precision clock signal during sleep mode, so that the microprocessor can be awakened to process an incoming telephone call or other data.
One simple implementation of circuitry for generation of a low power clock signal is shown in FIG. 1. FIG. 1 shows an integrated circuit 10 which contains auxiliary circuitry (not shown) such as the above-mentioned microprocessor, together with a pair of input terminals 11a and 11b. A crystal oscillator 12 is connected across the input terminals, and generates a sinusoidal wave at a frequency (for example) of 32 kHz. Clock signal generation circuitry includes an inverter 13 and an amplifier 14. Inverter 13 inverts the sinusoidal input, and the amplifier 14 is driven at high gain so as to convert the sinusoidal wave into a rail-to-rail square wave at the same frequency as that of crystal oscillator 12, i.e. at 32 kHz.
Although such circuitry desirably generates a clock signal with high slew rate, the circuit is not a low power circuit. Specifically, because the input sinusoidal wave is a slow-slewing signal, the circuitry shown in FIG. 1 will exhibit high crowbar current. As one example, assume that inverter 13 is implemented as a PMOS or NMOS transistor, or as a PMOS/NMOS pair of transistors. For such configurations, there will be a period of time during zero-crossing of the sinusoidal input signal where all transistors are “ON” and conducting full current (crowbar current) directly from source to ground. As a result, and since the circuitry of FIG. 1 is not a low power circuit, it is not suitable for use in many environments, particularly those where power management is important.
A common technique for reducing the power consumption of the circuitry shown in FIG. 1 is to run all initial stages of amplification from current minors. A simplified example of such an arrangement is shown in FIG. 2. FIG. 2 shows an integrated circuit 20 which, like FIG. 1, contains auxiliary circuitry (not shown) such as the above-mentioned microprocessor, together with a pair of input terminals 21a and 21b. Crystal oscillator 22 is connected across input terminals 21a and 21b. Inverter 23 is fed from a first current mirror 24, which budgets the amount of current that can be used by the inverter 23. The output of inverter 23 is provided to amplifier 25, which is fed by second current mirror 26 and which is driven at a high gain so as to convert the sinusoidal wave into a rail-to-rail square wave at the same frequency as that of crystal oscillator 22.